Field of the Invention
The present invention relates to wireless communication, and more particularly to a cell reselection method performed by a terminal in a wireless communication system and a terminal using the same.
Related Art
In International Telecommunication Union Radio communication sector (ITU-R), a standardization task for International Mobile Telecommunication (IMT)-Advanced, that is, the next-generation mobile communication system since the third generation, is in progress. IMT-Advanced sets its goal to support Internet Protocol (IP)-based multimedia services at a data transfer rate of 1 Gbps in the stop and slow-speed moving state and of 100 Mbps in the fast-speed moving state.
For example, 3rd Generation Partnership Project (3GPP) is a system standard to satisfy the requirements of IMT-Advanced and is preparing for LTE-Advanced improved from Long Term Evolution (LTE) based on Orthogonal Frequency Division Multiple Access (OFDMA)/Single Carrier-Frequency Division Multiple Access (SC-FDMA) transmission schemes. LTE-Advanced is one of strong candidates for IMT-Advanced.
There is a growing interest in a Device-to-Device (D2D) technology in which devices perform direct communication. In particular, D2D has been in the spotlight as a communication technology for a public safety network. A commercial communication network is rapidly changing to LTE, but the current public safety network is basically based on the 2G technology in terms of a collision problem with existing communication standards and a cost. Such a technology gap and a need for improved services are leading to efforts to improve the public safety network.
The public safety network has higher service requirements (reliability and security) than the commercial communication network. In particular, if coverage of cellular communication is not affected or available, the public safety network also requires direct communication between devices, that is, D2D operation.
D2D operation may have various advantages in that it is communication between devices in proximity. For example, D2D UE has a high transfer rate and a low delay and may perform data communication. Furthermore, in D2D operation, traffic concentrated on a base station can be distributed. If D2D UE plays the role of a relay, it may also play the role of extending coverage of a base station.
Meanwhile, a terminal may or may not be provided with a D2D resource for a D2D operation at each frequency. For example, a cell with a first frequency may provide a D2D resource, while a cell with a second frequency may not provide a D2D resource. Suppose that the terminal is currently camping on the cell with the second frequency and needs to camp on, for a D2D operation, the cell with the first frequency, which provides the D2D resource, due to restricted capability thereof. Then, the terminal needs to reselect the cell with the first frequency.
However, if a third frequency, which provides no D2D resource, has a higher priority than the first frequency, the terminal may not select the cell with the first frequency in the cell reselection process and thus may not perform the D2D operation.
That is, in the cell reselection process, if a frequency at which a resource for a D2D operation is assigned has a lower priority than a frequency at which a resource for a D2D operation is not assigned, the terminal may not perform a D2D operation.
Therefore, a cell reselection method considering a D2D operation of a terminal and a terminal using this method are necessary.